Fermentation is a chemical change induced by a living organism or enzyme, such as bacteria or the microorganisms occurring in unicellular plants, which involves the aerobic decomposition of hydrocarbons to produce a desired product along with carbon dioxide. Fermentation systems are used for the production of a large number of products such as antibiotics, vaccines, synthetic biopolymers, synthetic amino acids and edible proteins.
In conventional aerobic fermentation, air is supplied in large quantity to provide oxygen for respiration and growth. At the same time, carbon dioxide is stripped off by the remaining air that is not consumed by the biomass (bacteria, fungi, plant cells, etc.). Generally, the oxygen contained in the air bubbles must be dissolved in the broth before the biomass can consume it. Therefore, oxygen dissolution from air is a rate controlling factor. To maintain favorable air dissolution rate, the pressure of the fermenters are typically elevated to several atmospheres.
Increased productivity in a fermenter may involve increasing the concentration of the nutrient and biomass. Oxygen demand will accordingly increase in response to the additional nutrient and biomass concentration. More oxygen will be consumed if it is available. Therefore, supplying sufficient air (or oxygen) to the biomass is a major concern. At higher oxygen consumption rate, more carbon dioxide is produced. At some point, the level of carbon dioxide in the fermenter will poison the biomass and become a major problem in the fermentation process. This poisoning occurs when the amount of carbon dioxide being generated during respiration and growth of the biomass is faster than the removal rate. At a critical level, the excess dissolved carbon dioxide will retard the growth of the biomass. The critical carbon dioxide level is defined as the level of carbon dioxide in the fermentation vessel in which the carbon dioxide no longer serves a beneficial function in fermentation, but rather retards the growth of the biomass.
Since the carbon dioxide concentration in the exhaust of the fermenter is a much easier measurable value than dissolved carbon dioxide level within the fermenter, it has become an industrial standard to measure the carbon dioxide concentration in the exhaust. Therefore, each fermentation process has a certain predetermined critical carbon dioxide concentrate in the exhaust as a reference that the fermentation batches should not exceed. This critical carbon dioxide concentration in the exhaust has become a practical measurable limitation as one tries to increase the productivity or biomass concentration.
To increase productivity with higher biomass, it has been known in the art to increase the air flow. Increasing the air flow has the advantages of supplying extra oxygen to support denser biomass while stripping out more carbon dioxide. However, there is a practical limit as to the amount of air that can be introduced. Excess air will flood the impeller if the fermenter is mechanically agitated, thus rendering the agitator useless. In airlifted fermenters, it can also fluidize the broth or blow the content out of the fermenters. Therefore, an increase in air flow can only increase the productivity to a very small extent.
Other works have suggested the use of pure oxygen to supplement the air when the biomass concentration is high. However, it is believed that simply adding pure oxygen will work in fermentation only if the biomass is resistant to carbon dioxide poisoning. To the most part, the addition of pure oxygen will compound the problem since more carbon dioxide is being generated through respiration and growth of the biomass. Excess carbon dioxide will accumulate if the removal rate is not increased at a rate higher than the carbon dioxide production.
It has been known in the art to keep the biomass concentration low enough so that the carbon dioxide concentration in the exhaust (as a control method) will not exceed the critical value. Therefore, the carbon dioxide concentration in the exhaust is a limiting factor in productivity increase.
The art has only proposed solutions relating to increase oxygen dissolution rate while ignoring the effect of carbon dioxide poisoning. The prior art references provided for using oxygen in enrichment or direct injection, but none of them is believed to resolve the problems associated with carbon dioxide poisoning.
It is desirable, therefore, to provide a method for carrying out fermentation using oxygen which minimizes the effects of carbon dioxide poisoning.